X-ray crystallographic studies will be used to determine the atomic resolution structures of gag-encoded proteins and the assembly intermediates of HIV and the homologous RSV retroviruses. These will be combined with cryo-electron microscopy studies of self-assembled particles of Gag components. The results will be used for studies of antiviral agents that inhibit either assembly or uncoating of the retroviral cores and to clarify the pathway of their assembly.Crystals of HIV-1 capsid protein (CA) complexed with a cognate Fab fragment have recently yielded the structure this protein's N-terminal domain (two-thirds of the entire protein), whereas its C-terminal domain is largely disordered. The Fab molecules form a lattice that separates these C-terminal domains, thereby preventing them from associating to form higher order oligomers. The studies on HIV-1 CA will be augmented by the X-ray analyses of RSV-CA crystals, which contain a cylindrical self-assembled structure related to viral cores. Studies of various HIV-1 CA-NC fragments and assembled Gag particles are also in progress. Crystallization surveys of Gag-capsid proteins of homologous lenti equine infectious anemia virus, Maloney murine leukemia virus, and the distantly related spumavirus will be initiated for subsequent crystallographic analysis in an effort to identify structural determinants of assembled and conserved sites for antiviral compounds.